The present invention relates to the art of induction heating and more particularly to a unique compact induction heating system that is at least partially powered by a DC power source.
U.S. Pat. No. 6,237,576 is incorporated herein by reference to illustrate a fuel evaporation delivery system that can be used with the present invention.
Induction heating involves the use of an induction heating coil that is driven by alternating currents to induce voltage and thus current flow in a work piece encircled by or associated with the induction heating coil. Such technology has distinct advantages over convection heating, radiant heating, and conduction heating in that it does not require physical contact with the heated work piece or circulating gasses to convey combustion type heat energy to the work piece. Consequently, induction heating is clean, highly efficient, and usable in diverse environments. However, induction heating by work piece associated conductors normally involves power supplies connected to an AC line current. Such heating power supplies are constrained by the frequency of the incoming line. In some instances, the line voltage is three phase, which is rectified to produce a DC link and then converted to alternating current by use of an inverter.
Such DC link driven power supplies have two distinct disadvantages. They are relatively large and involve a heavy core that constitutes a major component of the input rectifier. Consequently, such power supplies cannot be fit into a small compartment, such as the area under the hood of a motor vehicle. Further, a heating system to be used in association with an internal combustion engine cannot involve induction heating, since there is no source of alternating current to drive the power supply for the induction heating coil.
The present invention overcomes the disadvantages associated with existing induction heating systems, wherein the system can be made quite compact so that it is capable of being located in a small compartment and/or which can be at least partially powered by a DC power source. The invention will be described with particular reference to an induction heating system that is located in a small compartment such as, but not limited to, the under hood of a motor vehicle or the cowling of other internal combustion engines. However, as can be appreciated, the invention has broader applications and can be used to heat any number of different substances or objects by being at least partially powered by a DC power source. Such applications include, but are not limited to, fluid heating (liquid water, ice, oil, fuel, lubricants, adhesives, cleaning fluids, various gasses or vapors, various other chemical compounds, etc.), soldering/brazing, shrink fitting, bonding/curing, air-guns, metal preheating, welding/cutting, replacing the uses of various torch applications, etc.
In one aspect of the present invention, there is provided a compact inverter having an at least partially clean DC input and components which fit into a relatively small housing with a volume of less than about 100 cubic inches. By developing a special induction heating system for use in a confined space, the advantages of induction heating can be employed for various heating functions (e.g., the confined space of an engine compartment, portable tools that involve heating, fuel cells, etc.). Consequently, the required heating operations in such a confined space can enjoy the advantages of induction heating with its efficiency, environmental friendly nature, and ease of control. In one embodiment, the DC input of the compact inverter is substantially a clean DC input. As defined herein, a clean DC input is a DC input that has not substantially been rectified thus having a minimal ripple factor that will adversely effect the operation of the high frequency inverter. Such clean DC inputs include, but are not limited to, batteries, fuel cells, solar power cells, etc. In one non-limiting example, a clean DC input is available in an implement or vehicle driven by an internal combustion engine, wherein the DC current is generated by an alternator and stored in a battery for use in the emission system of the internal combustion engine.
In accordance another and/or alternative aspect of the present invention, there is provided a compact induction heating system which utilizes a substantially clean source of DC current of less than about 100 volts. The system comprises an inverter such as, but not limited to, a high frequency inverter with an input connected to the DC source. A pair of AC tuning capacitors are connected in series across the clean DC source. Typically, the AC tuning capacitors are the same; however, the AC capacitors can be different. Each capacitor is initially charged to a portion of the input DC voltage. Typically, each capacitor is initially charged to half of the input DC voltage; however, each capacitor can be charged to different portions of the input DC voltage. The load inductor is connected at one end to the center junction of the two AC capacitors. A pair of solid state switches (e.g., IGBT transistors) are connected in series across the DC source and in parallel with the two series AC capacitors. The other end of the inductor is connected to the junction of the two switches. The switches are opened and closed (e.g., gated on and off) alternately at a frequency determined by the application (e.g., typically between about 5 kHz and about 30 kHz, but generally with a range capability of about 1 kHz to about 200 kHz; however, other ranges can be used.). The frequency of the gates can be equal to or different from the natural resonant frequency of the load. The power or the amount of heat generated can be varied by slightly adjusting the gating frequency above or below the natural resonant frequency of the load. When the first switch closes, the voltage stored in the first AC capacitor is discharged through the inductor, producing one half of the AC sinusoidal current, and back to the opposite polarity of the DC source. At the same time, the first capacitor is then charged to substantially the full potential of the DC source. The switch is then opened (turned off), and after a sufficient amount of dead time has elapsed (which dead time can be zero), the second switch is turned on. When the second switch is closed, the second AC capacitor discharges through the inductor, producing the other half of the AC sinusoidal current, and is then charged to substantially the full potential of the DC source, but in the opposite polarity of the other capacitor. This process is repeated as long as the gate signals are present. The subsequent cycles after the first cycle differ in the fact that the AC tuning capacitors are now charged to substantially the full potential of the DC input. The process is halted when the gating signals are removed or disabled. The AC current generated by the capacitor-transistor switching system (inverter) is passed though the inductor. This current induces a voltage within the part/work piece to be heated (via magnetic flux). The induced voltage develops a current within the part which meets resistance to the material which comprises the part. This resistance to current flow generates heat in the form of I2R losses, where (I) is the induced current and (R) is the resistance of the part. The heat developed in the part can be measured in watts (W). W=I2R. In one embodiment of the invention, the load inductor is typically the actual induction heating coil whereby the natural frequency of the two current paths is equal to the driven frequency of the switching circuit. In another and/or alternative embodiment of the invention, the single inductor is the primary of an output transformer so that the heat controlling driven frequency can be delivered to inductors that are smaller or larger than the nominal inductor. In still another and/or alternative embodiment, the compact induction heating system is used on an internal combustion engine driven implement having an engine driven alternator to generate DC current for storage in a battery used as a source of a substantially clean DC current of less than about 50 volts for vaporization and/or ignition of fuel in the engine. In one aspect of this embodiment, the DC current source is the alternator of the engine when the engine is driven and the battery of the engine when the internal combustion engine is not operating.
In accordance with still another and/or alternative aspect of the present invention, the clean DC voltage is generally up to about 50 volts DC, typically up to 24 volts DC, more typically greater than about 6 volts DC, even more typically about 12 to 24 volts DC, and still even more typically about 12 to about 20 volts DC In one embodiment of the invention, the power supply has a lower input limit of 6 volts DC In another and/or alterative embodiment of the invention, the inductor of the inverter is an induction heating coil. In still another and/or alterative embodiment of the invention, the inductor is a primary winding of an output transformer having a secondary winding forming the induction heating coil. In yet another and/or alterative embodiment of the invention, the frequency of the heating system can be as low as about 1 kHz, and is typically in the range of about 10-20 kHz to drastically reduce this size of those components constituting the inverter. By using high frequency control of the gating circuit, the housing for the inverter can be reduced to substantially less than about 100 cubic inches. This compact size allows the invertor to be used in a variety of small and/or compact spaces. In one aspect of this embodiment, the invertor is sized so as to easily fit under the hood of a motor vehicle or the cowling of an internal combustion driven implement. One of the advantages of an induction heating system of the type to which the present invention is directed is the ability to operate at a high frequency to produce a relatively low reference depth of heating by the output induction heating coil for efficient heating of related constituents within a very confined compartment.
In accordance with yet another and/or alternative aspect of the present invention, the gating circuit includes a two state counter with an adjustable oscillator to adjust the driven frequency to at least partially tune the actual output heating of the system. In this gating circuit, there are alternate gating pulses with an adjustable dead band between the pulses to operate the first and second switches.
In accordance with still yet another and/or alternative aspect of the present invention, there is a dead time between the pulses to allow the natural frequency of the two combined conductive paths to prepare for reversing of the switches.
The primary object of the present invention is the provision of a compact induction heating system that can be mounted in a confined area for diverse operations of induction heating in such confined areas.
Yet another and/or alternative object of the present invention is the provision of a compact induction heating system which is operated at a high frequency so that it can be mounted in a relatively small housing, such as a housing having a volume of less than about 100 cubic inches.
Still another and/or alternative object of the present invention is the provision of a compact induction heating system which utilizes a unique high frequency operated inverter for converting substantially clean DC current to high frequency heating current.
These and other objects and advantages will become apparent from the following description of the present invention utilizing the accompanying drawings.